I filled up two days ago and everything seemed fine, then when I was coming home from work monday I realized I had already gone through a 1/4 tank, this morning I dropped down to almost 1/2 on my way to work, about 13 miles.

My first thought was that someone was stealing gas, but I stayed at my girls place last night and was at mine before that, also I watched it go down all the way to work. I've been getting mid to high 14 mpg since I've owned the truck, about a year. Now itís around the 7.5-8mpgs so a HUGE decrease.

It's not the tires or winter gas, there has been little fluctuation since I put the tires on and the onset of winter gas, if it was either of those it would have happened long ago, I have done nothing to the truck in the last month or two. I have in the last six months changed the motor oil, diff oil, greased the joints, new air filter, cleaned the MAF and throttle body and replaced the spark plugs with ones form the dealer. I have no codes or pending codes. Not towing anything, nothing has changed in the last month or few days.

Any ideas? I've found a few suggestions around the web but mostly people saying it's winter gas or large aggressive tires, which is not the case since it hasn't been an issue for quite some time and suddenly dropped to almost half.

Did you gap the plugs? Don't know much at all about the 1st gens. My plugs were close from the dealer but not right on, one was actually almost touching. Idk? Wouldn't explain the crappy mileage all of a sudden. Good luck .

Did you gap the plugs? Don't know much at all about the 1st gens. My plugs were close from the dealer but not right on, one was actually almost touching. Idk? Wouldn't explain the crappy mileage all of a sudden. Good luck .

So my MPGs went back up to 13.5ish, still low but not nearly as bad. I figured my buddie would have something to test the back pressure, dude usually does. Rather then buying one I figure I could just take the cat off and run through a tank. Still haven't had time to do it though, not sure how loud it will be, I was thinking of fabbing up a through pipe.

So my MPGs went back up to 13.5ish, still low but not nearly as bad. I figured my buddie would have something to test the back pressure, dude usually does. Rather then buying one I figure I could just take the cat off and run through a tank. Still haven't had time to do it though, not sure how loud it will be, I was thinking of fabbing up a through pipe.

Another way to check your cat is by heat. Get a laser temp gauge. If the cat is hotter than the rest of your exhaust than its bad. They will glow cherry red sometimes when they are clogged. But a laser temp would tell you if there is a big difference.

Another way to check your cat is by heat. Get a laser temp gauge. If the cat is hotter than the rest of your exhaust than its bad. They will glow cherry red sometimes when they are clogged. But a laser temp would tell you if there is a big difference.

Another way to check your cat is by heat. Get a laser temp gauge. If the cat is hotter than the rest of your exhaust than its bad. They will glow cherry red sometimes when they are clogged. But a laser temp would tell you if there is a big difference.

I checked it yesterday, I left the numbers at the shop but it was hotter before the cat, the cat and pipe after were cooler. I think it was 450 before the cat and 350 at the cat and after.

This sounds like a cat issue to me too.
A good indication is really poor mpg and loss of power.
The power loss symptom is sometimes hard to determine.
If you have a light truck with good hp it may not show up easily.
One way to determine real power loss is to load the truck up, and hit a long steep grade. If u are depressing the gas and downshifting way too much, this is strike one.
The easy no high tech tool test before pulling cat is to remove the o2 sensor before the cat, tie it up, and hit the hill again. If u run right up the hill, strike 2.
Final test, pull the exhaust and look at the inside! It usually shows broken or chunked up pieces. Strike 3.
Time to post on TW for a used cat, hit the junkyard, or buy a new high proformance cat.
Good luck.